Friction stir welding is a method useful for joining together two separate metal pieces by way of a strong, permanent metallic bond (i.e., weld). The method involves rotating a friction stir welding tool at high speed and contacting the tool, with pressure, at a joint formed between edges of the metal pieces. Mechanical stirring and frictional heat introduced into the material at the joint by the rotating tool form the weld. A lengthwise joint is produced by moving the rotating tool along the joint.
Friction stir welding can be useful in manufacturing a metal product that includes two adjacent metal pieces that form a joint that requires a weld. As a single non-limiting example, friction stir welding may be useful for manufacturing electronic devices such as data storage devices (e.g., hard disk drives or HDDs). To increase reliability of certain types of electronic devices, their components may be placed within a metal enclosure that can be closed or sealed to prevent undesired moisture, gases, or particulates from contacting these components. The enclosure may preferably be hermetically sealed to create a compositionally stable interior atmosphere and to prevent gases from entering or exiting the interior. Enclosures of some hermetically-sealed electronic devices (e.g., hard disk drives) are filled with a non-air atmosphere, for example a low density atmosphere (e.g., helium), as a way to improve device performance. There are various benefits to operating a hard disk drive in a low density atmosphere. A low density atmosphere (relative to air, e.g., a helium atmosphere) in a hard disk drive can reduce the amount of drag force that affects a spinning disk or a stack of closely-spaced spinning disks. The reduced amount of drag can significantly reduce the amount of power required to cause the stack of disks to spin. Also, a low density atmosphere can reduce the amount of un-wanted, non-rotational movement or “flutter” that can occur in a spinning disk and a disk suspension during use. Reducing un-wanted motion of a disk or disk suspension can allow adjacent disks of a disk stack to be placed more closely together, which increases areal density (a measure of the quantity of information bits that can be stored on a given area of disk surface) by enabling a narrower data track pitch. Efficient thermal conduction of helium can also allow for a reduced operating temperature of a device, and a lower density gas (relative to air) can produce less acoustic noise. The reliability of a hard disk drive that contains helium in a hermetically sealed enclosure can also be increased due to a reduced level of humidity at the drive interior, due to elimination of other gases and contaminants such as corrosive gases, and due to a reduced sensitivity of the atmosphere to altitude and external pressure variations.
More generally, friction stir welding is useful for many purposes in addition to the manufacture of sealed enclosures of electronic devices. On a commercial scale, for friction stir welding to be a method of choice for welding two metal pieces together, the friction stir welding process must be efficient, cost-effective, reliable, and must produce a high quality weld that is strong and aesthetically suitable, meaning that the weld finish is smooth and preferably burr free. When the weld is produced on a hermetically sealed enclosure, the weld must be highly impermeable to gases. Manufacturers that require methods for welding two metal pieces together are in ongoing pursuit of new and improved welding tools and welding methods.